Controlling inductive power transfer systems

ABSTRACT

An inductive power transfer system ( 1 ) comprises a primary unit ( 10 ) operable to generate an electromagnetic field and at least one secondary device ( 30 ), separable from the primary unit, and adapted to couple with the field when the secondary device is in proximity to the primary unit so that power can be received inductively by the secondary device from the primary unit without direct electrical conductive contacts therebetween. The system detects if there is a substantial difference between, on the one hand, a power drawn from the primary unit and, on the other hand, a power required by the secondary device or, if there is more than one secondary device, a combined power required by the secondary devices. Following such detection, the system restricts or stops the inductive power supply from the primary unit. Such a system can detect the presence of unwanted parasitic loads in the vicinity of the primary unit reliably.

BACKGROUND OF THE INVENTION

The present invention relates to controlling inductive power transfersystems for use, for example, to power portable electrical or electronicdevices.

This application claims priority from the applicant's copendingapplications GB 0410503.7 filed on 11 May 2004 and GB 0502775.0 filed on10 Feb. 2005, the entire content of each of which is incorporated hereinby reference.

Inductive power transfer systems suitable for powering portable devicesmay consist of two parts:

-   -   A primary unit having at least one primary coil, through which        it drives an alternating current, creating a time-varying        magnetic flux.    -   A secondary device, separable from the primary unit, containing        a secondary coil. When the secondary coil is placed in proximity        to the time-varying flux created by the primary coil, the        varying flux induces an alternating current in the secondary        coil, and thus power may be transferred inductively from the        primary unit to the secondary device.

Generally, the secondary device supplies the transferred power to anexternal load, and the secondary device may be carried in or by a hostobject which includes the load. For example the host object may be aportable electrical or electronic device having a rechargeable batteryor cell. In this case the load may be a battery charger circuit forcharging the battery or cell. Alternatively, the secondary device may beincorporated in such a rechargeable cell or battery, together with asuitable battery charger circuit.

A class of such an inductive power transfer systems is described in ourUnited Kingdom patent publication GB-A-2388716. A notable characteristicof this class of systems is the physically “open” nature of the magneticsystem of the primary unit—a significant part of the magnetic path isthrough air. This is necessary in order that the primary unit can supplypower to different shapes and sizes of secondary device, and to multiplesecondary devices simultaneously. Another example of such an “open”system is described in GB-A-2389720.

Such systems may suffer from some problems. A first problem is that theprimary unit cannot be 100% efficient. For example, switching losses inthe electronics and I²R losses in the primary coil dissipate power evenwhen there is no secondary device present, or when no secondary devicesthat are present require charge. This wastes energy. Preferably theprimary unit should enter a low-power “standby mode” in this situation.

A second problem in such systems is that it is not possible tomechanically prevent foreign objects from being placed into proximitywith the primary coil, coupling to the coil. Foreign objects made ofmetal will have eddy-currents induced therein. These eddy currents tendto act to exclude the flux, but because the material has resistance, theflowing eddy currents will suffer I²R losses which will cause heating ofthe object. There are two particular cases where this heating may besignificant:

-   -   If the resistance of any metal is high, for example if it is        impure or thin.    -   If the material is ferromagnetic, for example steel. Such        materials have high permeability, encouraging a high flux        density within the material, causing large eddy currents and        therefore large I²R losses.

In the present application, such foreign objects that cause power drainare termed “parasitic loads”. Preferably the primary unit should enter a“shutdown mode” when parasitic loads are present, to avoid heating them.

Various approaches to solve these two problems have been proposed in theprior art.

Solutions to the first problem, of not wasting power when no secondarydevice requires charge, include:

-   -   In EP0533247 and U.S. Pat. No. 6,118,249 the secondary device        modulates its inductive load during charging, causing a        corresponding variation in the power taken from the primary        unit. This indicates to the primary unit that it should stay out        of the standby state.    -   In EP1022840 the primary unit varies the frequency of its drive,        and thus the coupling factor to a tuned secondary unit. If the        secondary unit is not taking power, there is no difference in        the power taken as the frequency is swept and thus the primary        unit goes into a standby state.    -   In U.S. Pat. No. 5,536,979 the primary unit simply measures the        power flowing in the primary coil, and enters a pulsing standby        state if this is below a threshold.    -   In U.S. Pat. No. 5,896,278 the primary unit contains detecting        coils which have power coupled back into them according to the        position of the secondary device. If the device is not present        the primary unit enters a standby mode.    -   In U.S. Pat. No. 5,952,814 the secondary device has a mechanical        protrusion which fits a slot in the primary unit, activating it.    -   In U.S. Pat. No. 6,028,413 the primary unit drives two coils,        and there are a corresponding two power receiving secondary        coils in the secondary unit. The primary unit measures the power        delivered from each primary coil and enters standby mode if it        is below a threshold.

Solutions to the second problem, of parasitic loads, include:

-   -   As mentioned above, in EP1022840 the primary unit varies the        frequency of its drive. In this system, the secondary device is        tuned, so this frequency variation will result in a variation of        the power taken from the primary unit. If the load is instead a        piece of metal, then varying the frequency will not have as much        effect and the primary unit will enter a shutdown state.    -   As mentioned above, in U.S. Pat. No. 5,952,814 a key in the        secondary device activates the primary unit. The assumption is        that if a secondary device is present then this will physically        exclude any foreign objects.    -   As mentioned above, in U.S. Pat. No. 6,028,413 the primary unit        supplies power to the secondary device by driving two primary        coils. If the amount of power supplied by the two coils is        different, the primary unit assumes that the load is not a valid        secondary device and enters shutdown mode.

These approaches all assume a 1:1 relationship between the primary unitand the secondary device. Therefore they are not sufficient for systemssuch as those described in GB-A-2388716 where more than one secondarydevice at a time may be present. For example, they would not work whenthere are two secondary devices present, one requiring charge and theother not.

Some of these approaches also assume that the physical or electricalpresence of a valid secondary device implies that all foreign objectsare physically excluded by the secondary device. This is not necessarilythe case, particularly when the secondary devices may be positionedarbitrarily in respect of the primary unit, as in those described inGB-A-2388716.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided amethod of controlling inductive power transfer in an inductive powertransfer system comprising a primary unit operable to generate anelectromagnetic field and at least one secondary device, separable fromthe primary unit, and adapted to couple with said field when thesecondary device is in proximity to the primary unit so that power canbe received inductively by the secondary device from the primary unitwithout direct electrical conductive contacts therebetween, which methodcomprises: setting the or each secondary device to a no-load state inwhich supply of any of the power received inductively by the secondarydevice to an actual load thereof is substantially prevented; and in theprimary unit, measuring a power drawn from the primary unit when the oreach secondary device is set into said no-load state, and restricting orstopping the inductive power supply from the primary unit in dependenceupon the measured power.

Because the secondary device(s) are set into the no-load state duringthe power measurement, it can be detected easily from the measured powerif there is a substantial parasitic load. If so, the primary unit canenter a shutdown mode. For example, the if the measured power is greaterthan a threshold value in the no-load state, then the power supply maybe restricted or stopped.

This method is convenient because it is unnecessary for the secondarydevices to communicate their power requirements to the primary unit orfor the primary unit to carry out any summation of the powerrequirements: it is known that since the secondary devices are in theno-load state their combined power requirement is zero or at least asmall value influenced by any parasitic load imposed on the primary unitby the secondary devices themselves.

According to a second aspect of the present invention there is provideda method of controlling inductive power transfer in an inductive powertransfer system comprising a primary unit operable to generate anelectromagnetic field and at least one secondary device, separable fromthe primary unit, and adapted to couple with said field when thesecondary device is in proximity to the primary unit so that power canbe received inductively by the secondary device from the primary unitwithout direct electrical conductive contacts therebetween, which methodcomprises: in the primary unit, receiving, from the or each secondarydevice that is in a power-requiring state, information relating to apower requirement of the secondary device concerned; and in the primaryunit, measuring the power drawn from the primary unit when power isbeing supplied to the or each secondary device having thepower-requiring state, and restricting or stopping the inductive powertransfer from the primary unit in dependence upon the measured power andthe received power requirement information.

In this case, the inductive power supply from the primary unit may berestricted or stopped in dependence upon a difference between saidmeasured power and a sum of the respective power requirements of thesecondary devices having said power-requiring state. For example, theinductive power supply may be restricted or stopped in the event thatthe measured power exceeds said sum by more than a threshold value.

This method has the advantage over the method of the first aspect thatit does not need to set the secondary devices into the no-load stateduring the power measurement. Thus, power can be supplied continuouslyto the actual loads of the secondary devices.

Of course, in the method of the first aspect, the power measurementperiod can be very short, so that any interruption to the power supplyto the load may be unnoticeable. If interruption of the load is aproblem, it is possible to provide an energy storage means such as acapacitor in the secondary device to maintain the power supply to theactual load during the power measurement period.

In the method of the second aspect, any suitable communication methodmay be used to transmit the power requirement information from eachsecondary device to the primary unit. One preferred method for the oreach secondary device to transmit its power requirement information tothe primary unit is an RFID method. Alternatively, the or each secondarydevice may transmit its power requirement information to the primaryunit by varying a load imposed by the secondary device on the primaryunit.

It will be appreciated that the methods embodying the first and secondaspects of the invention provide different ways of detecting if there isa substantial difference between, on the one hand, a power drawn fromthe primary unit and, on the other hand, a power required by thesecondary device or, if there is more than one secondary device, acombined power required by the secondary devices. Following suchdetection, the inductive power supply from the primary unit can berestricted or stopped.

In the methods of the first and second aspects it is possible to vary aload imposed by the secondary device on the primary unit to communicatea signal or information from the secondary device to the primary unit.For example, the power requirement information needed in the secondaspect may be transmitted this way.

An advantage of communicating using load variation is that it can permittwo or more, or possibly all, secondary devices to supply respectiveitems of information simultaneously to the primary unit. For example, ifany secondary device requires power it may vary its load. If the primaryunit detects no or no substantial overall load variation it can concludethat no secondary device requires power and hence enter a standby mode.Similarly, the primary unit will detect the sum of any load variations.If the load variation from each individual secondary device isproportional to some analog quantity to be communicated to the primaryunit, for example the power requirement or parasitic load of thesecondary device, then the sum of the respective analog quantities willbe detected by the primary unit in the power measurement. This meansthat the sum can be obtained directly without the need for additionalprocessing in the primary unit which may be time-consuming and/or costlyto implement.

It may also be desirable to detect a condition for entering a standbymode in addition to detecting when to enter the shutdown mode. Forexample, in the method of the first aspect it is possible to restrict orstop the inductive power supply in the event that the measured power isless than a standby threshold value (different from the shutdownthreshold mentioned above). Another possibility is that the or each saidsecondary device reports to the primary unit state informationindicating whether the secondary device is in a no-power-requiringstate, in which an actual load of the secondary device currentlyrequires no power from the primary unit, or a power-requiring state inwhich said active load does currently require power from the primaryunit. The primary unit then restricts or stops the inductive powersupply therefrom in dependence upon the state information reported bythe or each secondary device. For example, the primary unit may restrictor stop the inductive power transfer unless the state informationreported by at least one secondary device indicates that it has saidpower-requiring state. Preferably, for speed of response, two or moresecondary devices report their respective state informationsimultaneously to the primary unit. One convenient possibility, as notedabove, is for the or each said secondary device to report its said stateinformation by varying a load imposed by it on the primary unit.

In general, it is possible to carry out two or more measurements of thepower drawn from the primary unit in different measurement periods. Ifthe secondary devices are synchronised with the primary unit, then theycan behave differently in one measurement period from another, so thatthe primary unit can detect two or more different conditions under whichpower restriction or stopping is appropriate.

One preferred embodiment has three measurement periods. In the firstperiod, each secondary device turns off a dummy load. In the secondperiod, each secondary device that requires power turns on its dummyload. The other secondary devices turn their dummy loads off. In thethird period, each secondary device turns on its dummy load. The primaryunit can detect from a comparison of the power measurements in the threeperiods whether there is a substantial parasitic load requiring shutdownor no device requiring power so that standby is appropriate.

It is also possible to vary the load during a measurement period. Forexample, the amplitude of the load change may be fixed but the durationmay be varied to supply information.

The primary unit may have registered therein a power requirement of atleast one said secondary device. In this case the power requirementinformation transmitted from that secondary device may simply beinformation identifying the secondary device. The primary unit employsthe identifying information to retrieve the registered power requirementfor the device. The identifying information can be a code or a type,model or serial number assigned to the secondary device. This can reducethe amount of information to be transmitted to the primary unit and alsoimprove speed of response and reliability.

Although each secondary device that is in the power-receiving statetransmits power requirement information to the primary unit in themethod of the second aspect, the or each said secondary device nothaving the power-requiring state may also transmit such powerrequirement information to the primary unit, if desired. One possibilityis for the power requirement information transmitted by the or each saidsecondary device not in said power-requiring state to be representativeof a parasitic load imposed on the primary unit by the secondary device.This can then be used to make the shutdown detection more reliable. Itis also possible for the power requirement information to be the sum ofpower requirement of the actual load and the parasitic load of thesecondary device when in the power-requiring state and just the powerrequirement of the parasitic load when not in that state.

Generally, it is desirable for the detection of the conditions forrestricting or stopping the inductive power supply to take account ofany losses in the primary unit and the secondary devices. There are anumber of ways of doing this.

One way is to employ, when carrying out said detection, firstcompensation information relating to losses in the primary unit itselfso as compensate for said losses. It is possible to derive part or allof said first compensation information from measurements taken by theprimary unit when it is effectively in electromagnetic isolation. Thefirst compensation information may be stored in a calibration unit ofthe primary unit.

Another way is to employ, when carrying out said detection, secondcompensation information relating to a parasitic load imposed on theprimary unit by the or each secondary device so as to compensate forsaid parasitic load of the or each secondary device. The or each saidsecondary device preferably communicates its said second compensationinformation directly to said primary unit or communicates to the primaryunit other information from which the primary unit derives said secondcompensation information. The secondary device may communicate its saidsecond compensation information or its said other information to theprimary unit by varying a load imposed by it on the primary unit, asmentioned above.

A particularly convenient and efficient way is for the or each saidsecondary device to have a dummy load, representative of its saidparasitic load, which it imposes on said primary unit to vary the loadimposed by it on the primary unit.

Part or all of said first compensation information and/or part or all ofsaid second compensation information may be information stored in theprimary unit during manufacture and/or testing of the primary unit.

It may be advantageous to vary one or both of said first and secondcompensation information when one or more operating conditions (e.g.temperature) of the primary unit varies. A secondary device may becapable of being used either by itself or combination with anotherobject. For example, the secondary device may be removable from a hostobject. If it can be powered when removed or when installed in the hostobject, the parasitic load of the device alone is likely to be quitedifferent from the parasitic load of the device and host objecttogether. To deal with this situation, the second compensationinformation may be varied in dependence upon whether the device is usedby itself or in combination with another object.

In many implementations, the secondary devices need to operate insynchronism with primary unit. It is therefore preferable to transmit asynchronising signal from the primary unit to the or each said secondarydevice, or from the or each said secondary device to the primary unit,to synchronise operation of the primary unit and the or each saidsecondary device. This is conveniently done by modulating a drive signalapplied to a primary coil in the primary unit. Frequency, amplitude orphase modulation, or a combination thereof, may be used.

Many different techniques can be used to measure the power drawn fromthe primary unit by the secondary devices. In one technique, theelectromagnetic field is generated by a primary coil driven by anelectrical drive unit, and electrical power for the drive unit issupplied from a power supply of the primary unit to a power input of thedrive unit. The power drawn from the primary unit is measured bytemporarily disconnecting the power supply and detecting a change atsaid power input during the disconnection. The change may be a voltagedecay. The advantage of this technique is that there is no seriesresistance through which the current for drive unit passes. Such aseries resistance dissipates significant power.

It is preferable to store energy in an energy storage unit such as acapacitor connected to said power input so that power can continue to besupplied to said power input whilst said power supply is disconnected.

Another way to measure the power drawn may be available if theelectrical drive unit has a feedback circuit to control the current orpower of the drive to the primary coil in this case, a feedback signalin the feedback circuit may provide a measure of the power being drawnwithout the need to add in a power measurement unit at all.

Another way to measure the power involves causing a circuit includingsaid primary coil to operate, during a measurement period, in anundriven resonating condition in which the application of drive signalsto the primary coil is suspended so that energy stored in the circuitdecays over the course of said period. One or more measures of suchenergy decay are then taken during said period and employed to measuresaid power drawn from the primary unit.

Two or more power measurements may be taken under the same conditionsand the results averaged to improve accuracy.

In operation it may be desirable to set the field magnitude of theelectromagnetic field to a value lower than the maximum value even inthe operating mode. In the method of the second aspect the primary unithas the power requirement information from each secondary device and cantherefore easily set the field magnitude in dependence upon the powerrequired by the secondary device or, if there is more than one secondarydevice, the combined power required by the secondary devices. In thisway a minimum power output for powering the secondary devices can befound. However, there are other ways to achieve a similar result. Forexample, a secondary device that is not getting enough power maymodulate its load in some way. The primary unit may start operating atmaximum power and reduce the power until the load modulation is detectedfrom at least one secondary device. This enables minimum power to bedetermined in a simple and quick way.

According to a third aspect of the present invention there is providedan inductive power transfer system comprising: a primary unit operableto generate an electromagnetic field; at least one secondary device,separable from the primary unit, and adapted to couple with said fieldwhen the secondary device is in proximity to the primary unit so thatpower can be received inductively by the secondary device from theprimary unit without direct electrical conductive contacts therebetween;means for detecting if there is a substantial difference between, on theone hand, a power drawn from the primary unit and, on the other hand, apower required by the secondary device or, if there is more than onesecondary device, a combined power required by the secondary devices;and means operable, following such detection, to restrict or stop theinductive power supply from the primary unit.

According to a fourth aspect of the present invention there is provideda primary unit, for use in an inductive power transfer system that alsohas at least one secondary device separable from the primary unit, theprimary unit comprising: means for generating an electromagnetic fieldwhich couples with said at least one secondary device when it is inproximity to the primary unit so that power can be received inductivelyby the secondary device from the primary unit without direct electricalconductive contacts therebetween; means for detecting if there is asubstantial difference between, on the one hand, a power drawn from theprimary unit and, on the other hand, a power required by the secondarydevice or, if there is more than one secondary device, a combined powerrequired by the secondary devices; and means operable, following suchdetection, to restrict or stop the inductive power supply from theprimary unit.

According to a fifth aspect of the present invention there is provided asecondary device, for use in an inductive power transfer system thatcomprises a primary unit which generates an electromagnetic field, thesecondary device comprising: a secondary coil adapted to couple withsaid field generated by said primary unit when the secondary device isin proximity to the primary unit so that power can be receivedinductively by the secondary device from the primary unit without directelectrical conductive contacts therebetween; load connection means,connected to said secondary coil and adapted to be connected when thesecondary device is in use to a load requiring power from the primaryunit, for supplying such inductively-received power to the load;detecting means for detecting a synchronisation signal transmitted bythe primary unit; and control means, responsive to the detection of thesynchronisation signal, to set the secondary device into a no-load statein which supply by the load connection means of any of theinductively-received power to said load is substantially prevented.

This can provide a secondary device adapted for use in the methodembodying the first aspect of the invention described above.

According to a sixth aspect of the present invention there is provided asecondary device, for use in an inductive power transfer system thatcomprises a primary unit which generates an electromagnetic field, thesecondary device comprising: a secondary coil adapted to couple withsaid field generated by said primary unit when the secondary device isin proximity to the primary unit so that power can be receivedinductively by the secondary device from the primary unit without directelectrical conductive contacts therebetween; load connection means,connected to said secondary coil and adapted to be connected when thesecondary device is in use to a load requiring power from the primaryunit, for supplying such inductively-received power to the load; andRFID communication means operable to supply to the primary unit, usingan RFID communication method, information relating to a powerrequirement of the secondary device.

This can provide a secondary device adapted for use in the methodembodying the second aspect of the invention described above. In thiscase the load connection means does not disconnect the actual loadduring the power measurement.

According to a seventh aspect of the present invention there is provideda method of controlling conductive power transfer in an inductive powertransfer system comprising a primary unit operable to generate anelectromagnetic field and at least one secondary device, separable fromprimary unit, and adapted to couple with said field when the secondarydevice is in proximity to the primary unit so that power can be receivedinductively by the secondary device from the primary unit without directelectrical conductive contacts therebetween, which method comprises: inan information supplying phase, permitting two or more secondary devicesto supply simultaneously to the primary unit information relatingrespectively to the secondary devices concerned; and interpreting thesimultaneously-supplied information at the primary unit and determiningbased on the interpreted information whether to restrict or stop theinductive power supply from the primary unit.

This method can permit rapid supply of information or signals from thesecondary devices to enable restriction or stopping of the power supplyto be achieved quickly.

In one embodiment, the information supplied from each secondary deviceindicates whether or not the secondary device concerned is in apower-requiring state in which an actual load of the secondary devicerequires power from the primary unit, and the primary unit determinesthat the inductive power supply therefrom should be restricted orstopped unless the information supplied in said information supplyingphase by at least one of the secondary devices indicates that it hassaid power-receiving state.

The information supplied from each secondary device may represent ananalog quantity of the secondary device concerned. In this case, theprimary unit can derive directly from the simultaneously-suppliedinformation a sum of the respective analog quantities of the secondarydevices.

The analog quantity may be representative of a parasitic load imposed onthe primary unit by the secondary device itself.

The analog quantity may be representative of a power requirement of anactual load of the secondary device.

The analog quantity may be representative of a total load imposed on theprimary unit by the secondary device, said total load including anactual load of the secondary device and a parasitic load imposed on theprimary unit by the secondary device itself.

In one embodiment, each said secondary device supplies its saidinformation by varying a load imposed by it on the primary unit. Forexample, each said secondary device may have a dummy load which itimposes selectively on the primary unit during said informationsupplying phase. The dummy load is preferably representative of saidanalog quantity. Different dummy loads may be used to representdifferent analog quantities, for example a power requirement and aparasitic load.

In one embodiment each said secondary device has its said informationsupplying phase at a time determined by the primary unit.

According to an eighth aspect of the present invention there is provideda method of controlling inductive power transfer in an inductive powertransfer system comprising a primary unit operable to generate anelectromagnetic field and at least one secondary device, separable fromthe primary unit, and adapted to couple with said field when thesecondary device is in proximity to the primary unit so that power canbe received inductively by the secondary device from the primary unitwithout direct electrical conductive contacts therebetween, in whichmethod: in a reporting phase the or each said secondary device reportsto the primary unit information indicating whether the secondary deviceis in a no-power-requiring state, in which an actual load of thesecondary device currently requires no power from the primary unit, or apower-requiring state in which said active load does currently requirepower from the primary unit; and the primary unit determines that theinductive power supply therefrom should be restricted or stopped independence upon the information reported by the or each secondary devicein said reporting phase.

Preferably, the or each said secondary device has its said reportingphase at a time determined by the primary unit.

In one embodiment there are at least two secondary devices and each saidsecondary device has its said reporting phase at the same time.

The or each said secondary device may report its said information byvarying a load imposed by it on the primary unit. For example, the oreach said secondary device may have a dummy load which it imposesselectively on the primary unit during its said reporting phase.

In one embodiment, the or each said secondary device that has saidpower-requiring state imposes its said dummy load during said reportingphase and the or each said secondary device that has saidnon-power-requiring state does not impose its said dummy load duringsaid reporting phase.

According to a ninth aspect of the present invention there is provided asecondary device, for use in an inductive power transfer system thatcomprises a primary unit which generates an electromagnetic field, thesecondary device comprising: a secondary coil adapted to couple withsaid field generated by said primary unit when the secondary device isin proximity to the primary unit so that power can be receivedinductively by the secondary device from the primary unit without directelectrical conductive contacts therebetween; load connection means,connected to said secondary coil and adapted to be connected when thesecondary device is in use to a load requiring power from the primaryunit, for supplying such inductively-received power to the load; andcommunication means operable to communicate to the primary unitinformation relating to a parasitic load imposed on the primary unit bythe secondary device.

Such a secondary device can communicate its parasitic load to theprimary unit for the primary unit to use to compensate for that load.For example, the communicated parasitic load can be used when detectingconditions for restricting or stopping inductive power transfer from theprimary unit.

Any communication method can be used, and the method is not limited toload variation. For example, infrared or ultrasonic communication can beused. RFID can also be used.

In one embodiment, said communication means are operable to communicatesaid information by imposing a dummy load on said primary unit. Thecommunication means may be operable to impose a first dummy load on theprimary unit at a first time and a second dummy load, different fromsaid first dummy load, at a second time, a difference between said firstand second dummy loads being set in dependence upon said parasitic load.One of said first and second dummy loads may be zero.

According to a tenth aspect of the present invention there is provided aportable electrical or electronic device comprising: a load which atleast at times requires power from said primary unit; and a secondarydevice embodying the aforesaid fifth, sixth or ninth aspect of thepresent invention, said load connection means of said secondary devicebeing connected to said load for supplying such inductively-receivedpower to the load at said times.

According to an eleventh aspect of the present invention there isprovided a method of controlling inductive power transfer in aninductive power transfer system comprising a primary unit, having aprimary coil to which electrical drive signals are applied to generatean electromagnetic field, and also comprising at least one secondarydevice, separable from the primary unit and having a secondary coiladapted to couple with said field when the secondary device is inproximity to the primary unit so that power can be transferredinductively from the primary unit to the secondary device without directelectrical conductive contacts therebetween, which method comprises:causing a circuit including said primary coil to operate, during ameasurement period, in an undriven resonating condition in which theapplication of said drive signals to said primary coil is suspended sothat energy stored in the circuit decays over the course of said period;taking one or more measures of such energy decay during said period; andrestricting or stopping inductive power transfer from the primary unitin dependence upon said one or more energy decay measures.

Such a method can enable either or both of parasitic load and standbydetection to be achieved in a robust and cost-effective manner. It isparticularly advantageous in systems which may have multiple secondarydevices and/or whose open magnetic nature makes it easy for parasiticobjects to couple to the primary coil.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made, by way of example, to the accompanyingdrawings in which:

FIG. 1 is a block diagram showing parts of an inductive power transfersystem embodying the present invention;

FIG. 2 is a flowchart for use in explaining a first method of detectinga shutdown condition in accordance with the present invention;

FIG. 3 is a flowchart for use in explaining a first method of detectinga standby condition in accordance with the present invention;

FIG. 4 is a block diagram showing parts of an inductive power transfersystem according to a first embodiment of the present invention;

FIG. 5 shows waveform diagrams for use in explaining operation of theFIG. 4 system;

FIGS. 6 show waveform diagrams showing the timing of the various signalswithin the FIG. 4 system: FIG. 6( a) shows a frequency of an AC voltagesignal applied to a primary coil; FIG. 6( b) shows a power drawn fromthe primary unit; FIG. 6( c) shows a state of a switch in the primaryunit; and FIG. 6( d) shows the voltage across the switch in the primaryunit;

FIG. 7 is a diagram showing the load drawn during three differentmeasurement operations;

FIG. 8 is a diagram illustrating different modes of operation in theFIG. 4 system;

FIG. 9 is a block diagram showing parts of a primary unit in a powertransfer system according to a second embodiment of the presentinvention;

FIG. 10 shows how current flowing through a primary coil varies innormal, snub and decay states which occur during a power measurement inthe FIG. 9 system:

FIG. 11 is a flowchart for use in explaining a second method ofdetecting a shutdown condition in accordance with the present invention;and

FIG. 12 is a block diagram showing parts of a power transfer systemaccording to a third embodiment of the present invention.

DESCRIPTION OF THE CURRENT EMBODIMENTS

FIG. 1 illustrates parts of an inductive power transfer system embodyingthe present invention. The system 1 comprises a primary unit 10 and atleast one secondary device 30. The primary unit 10 has a primary coil 12and an electrical drive unit 14 connected to the primary coil 12 forapplying electrical drive signals thereto so as to generate anelectromagnetic field. A control unit 16 is connected to the electricaldrive unit 14. This control unit generates an AC voltage signal 106. Theelectrical drive unit takes the AC voltage signal 106 and converts it toan AC current signal in the primary coil 12, so as to generate aninduced electromagnetic field in the proximity of the primary coil 12.

The primary unit 10 may have any suitable form but one preferred form isa flat platform having a power transfer surface on or in proximity towhich each secondary device 30 can be placed. In this case, the fieldmay be distributed over a power transfer area of the surface, asdescribed in GB-A-2388716.

The secondary device 30 is separable from the primary unit 10 and has asecondary coil 32 which couples with the electromagnetic field generatedby the primary unit 10 when the secondary device 30 is in proximity tothe primary unit 10. In this way, power can be transferred inductivelyfrom the primary unit 10 to the secondary device 30 without directelectrical conductive contacts therebetween.

The primary coil 12 and the secondary coils 32 can have any suitableforms, but may for example be copper wire wound around ahigh-permeability former such as ferrite or amorphous metal.

The secondary device 30 is usually connected to an external load (notshown—also referred to herein as the actual load of the secondarydevice) and supplies the inductively-received power to the externalload. The secondary device 30 may be carried in or by an objectrequiring power such as a portable electrical or electronic device or arechargeable battery or cell. Further information regarding designs ofsecondary device 30 and the objects which can be powered using thesecondary devices 30 can be found in GB-A-2388716.

The primary unit 10 in the FIG. 1 system also comprises a powermeasurement unit 100 connected to the control unit 16. The powermeasurement unit 100 performs a measurement of the electrical powerdrawn by the electrical drive unit 14, on receipt of a signal providedby the control unit 16. The power measurement unit 100 provides anoutput representative of the electrical power drawn by the electricaldrive unit 14 to the control unit 16. The power drawn by the electricaldrive unit 14 is representative of the power drawn by the primary coil12 and hence also the power drawn by all the secondary devices 30 plusother losses.

In the FIG. 1 system it is desirable to detect certain conditions andrestrict or stop the inductive power supply from the primary unit underthose conditions.

One such condition is the presence of a substantial parasitic load inthe vicinity of the primary unit. In this case, the control unit 16 mayenter a shutdown mode in which the drive to the primary coil 12 isreduced or stopped, preventing heating of the parasitic load.

Another such condition is when no secondary device 30 of the system ispresent in the vicinity of the primary unit 10. Another such conditionis when there is at least one secondary device 30 present but none ofthe devices has a load currently requiring power. A load does notrequire power, for example, when turned off or when, in the case of arechargeable battery or cell, the battery or cell is fully charged.Under both these conditions the control unit 16 may enter a standby modein which the drive to the primary coil 12 is reduced or stopped,preventing unnecessary power consumption in the primary unit 10.

FIG. 2 is a flowchart for use in explaining a first method for detectingthe presence of a substantial parasitic load in the vicinity of theprimary unit in accordance with the present invention.

In this first method, when the system of FIG. I is in use, from time totime all of the secondary devices in the vicinity of the primary unitare deliberately set to a no-load state. In this no-load state, supplyof any of the power received inductively by the secondary device to anactual load thereof (the external load mentioned above) is prevented.

In step S2, with all of the secondary devices in the no-load state, thepower measurement unit 100 in the primary unit measures the power drawnby the secondary devices from the primary unit. In step S3, the controlunit 16 in the primary unit determines, in dependence upon the powermeasured in step S2, whether to restrict or stop the inductive powersupply from the primary unit.

In the simplest case, in step S3 the control unit 16 simply compares themeasured power with a predetermined shutdown threshold. If the measuredpower exceeds the shutdown threshold value, the control unit 16determines that the inductive power supply from the primary unit shouldbe restricted or stopped. As described in more detail below, however, itis preferable to take into account losses which inevitably occur in thepower transfer system. In particular, these losses include losses in theprimary unit itself and/or any secondary devices/host objects present.These losses include inefficiencies in the primary coil itself and anyother components associated with the primary coil such as the electricaldrive unit, for example I²R losses in the copper of the coil oreffective series resistance of any resonating capacitor. The losses alsoinclude any magnetic losses in the primary unit and the secondarydevices, for example magnetic is hysteretic loop losses in any coilsassociated with the primary unit and/or secondary devices. According,the control unit 16 may employ, in addition to the measured power, firstcompensation information relating to losses in the primary unit itself,so as to compensate for those losses in step S3. Alternatively, or inaddition, the control unit 16 may employ, in addition to the measuredpower, second compensation information relating to a parasitic loadimposed on the primary unit by the or each secondary device, so as tocompensate for the parasitic load of the or each secondary device instep S3.

If it is determined in step S3 that the power supply should berestricted or stopped, then in step S4 the control unit 16 places theprimary unit in a shutdown mode in which the inductive power supply fromthe primary unit is restricted or stopped.

The primary unit will remain in the shutdown mode until it is reset insome way. Such a reset could be manually initiated by a user of theprimary unit, or alternatively the control unit 16 could periodicallystart to supply inductive power again and repeat steps S1 to S3 todetermine whether to remain in the shutdown mode or not.

In step S3, if the control unit 16 determines that the power supply doesnot need to be restricted or stopped, then in step S6 secondary devicesrequiring power resume receiving power from the primary unit. Processingthen returns, for example after a predetermined interval, to step S1again.

Next, a first method of detecting the conditions for entering thestandby mode will be described with reference to FIG. 3.

In FIG. 3, as shown in step S11, from time to time each secondary device(if any) that is present in the vicinity of the primary unit 10 has areporting phase. All of the secondary devices which are present mayenter a reporting phase simultaneously. Alternatively, each of thesecondary devices in turn may enter the reporting phase individually. Ineither case, in the reporting phase each secondary device reports to theprimary unit state information indicating whether the secondary deviceis in a no-power-requiring state or a power-requiring state. In theno-power-requiring state, an actual load of the secondary devicecurrently requires no power from the primary unit. In thepower-requiring state, on the other hand, the actual load does currentlyrequire power from the primary unit.

In step S12 the control unit 16 in the primary unit determines if theinductive power supply therefrom should be restricted or stopped independence upon the state information reported in step S11. Inparticular, unless at least one secondary device reports in thereporting phase to the primary unit that it is in the power-requiringstate the control unit 16 determines that the inductive power supplyshould be restricted or stopped and processing proceeds to step S13 inwhich the primary unit is set into the standby mode. Of course, if nosecondary device is present in the vicinity of the primary unit at all,so that in step S11 no or no valid state information is received by theprimary unit, then the control unit 16 also sets the primary unit intothe standby mode.

As described above in relation to step S5 of the method of FIG. 2, oncethe primary unit has been set into the standby mode it may be reset intoan operating mode again either by manual intervention from a user orautomatically.

If in step S12 the control unit 16 determines that the inductive powersupply should not be restricted or stopped based on the reported stateinformation, then processing returns to step S11, for example after apredetermined interval. In this way, each secondary device presentperiodically has a reporting phase in which it reports its stateinformation to the primary unit.

The methods of FIGS. 2 and 3 may be carried out independently of oneanother. However, it is preferable for the control unit 16 of theprimary unit to be able to detect both when to enter the shutdown modeand to enter the standby mode. This can be achieved by a combination ofthe methods of FIGS. 2 and 3 as will now be described with reference toFIG. 4.

FIG. 4 shows parts of an inductive power transfer system according to afirst embodiment of the present invention. The system 1 has a primaryunit 10 and a secondary device 30. FIG. 4 also shows a parasitic load500 on the primary unit, caused for example by a foreign object placedin the vicinity of the primary unit 10. The secondary device 30 in thiscase is assumed to be carried in or by a host object such as a portableelectrical or electronic device. As explained hereinbefore the secondarydevice 30 and/or host object also inevitably impose a “friendly”parasitic load 501 on the primary unit 10.

As described earlier with reference to FIG. 1, the primary unit 10comprises a primary coil 12, an electrical drive unit 14, a control unit16 and a power measurement unit 100. The electrical drive unit 14 has aninput connected to an output of the control unit 16 supplying the ACvoltage signal 106. The output nodes of the electrical drive unit 14 areconnected to the primary coil 12.The electrical drive unit is connectedto the power supply 105, via the power measurement unit 100. The powersupply 105 supplies direct current to the electrical drive unit 14. Theelectrical drive unit 14 presents a high input impedance for the ACvoltage signal 106, so that essentially all the load current is drawnfrom power supply 105.

The control unit 16 is a microprocessor in this embodiment. Themicroprocessor has an inbuilt digital-to-analogue converter (not shown)to drive the output supplying the AC voltage signal 106. Alternatively,an ASIC could be used to implement the control unit 16, as well as someor all of the other circuit elements of the primary unit.

The control unit 16 in this embodiment is adapted to modulate the ACvoltage signal 106 for transmitting a synchronisation signal to asecondary device. The modulation is a frequency modulation of the ACvoltage signal. Other modulation techniques such as amplitude or phasemodulation may also be used. The control unit 16 is adapted to send asynchronisation signal to any secondary devices 30 present. Thesecondary devices 30 alter their load conditions in response to thesynchronisation signal. This information is used to detect theconditions for entering the shutdown and standby modes.

It is desirable for the power measurement unit 100 to be operativewithout the need to disconnect the electrical supply to the primary coil12, as this means that supply to the secondary device 30 is notinterrupted and it reduces stray electromagnetic interference into thesurrounding environment. This is challenging because there is a lot ofnoise and the measurement is required in a short time.

The power measurement unit 100 comprises a switch 102 between the 0Vsupply in the power supply 105 and the ground terminal of the electricaldrive unit 14. The switch 102 is controlled by the control unit 16. Thepower measurement unit also comprises a capacitor 101 connected betweenthe positive and ground terminals of the electrical drive unit 14. Thecapacitor functions as an energy storage unit. There is a differentialamplifier 103 with inputs either side of the switch 102 which has anoutput coupled to an analogue-to-digital converter 104. Theanalogue-to-digital converter output is coupled to the control unit 16.

When the switch 102 is closed, the power measurement unit 100 is notoperative and the power is directly coupled from the power supply 105 tothe electrical drive unit 14. A power measurement is performed when theswitch 102 is opened. The capacitor 101 is now disconnected from the 0Vrail on the power supply 105, but still retains its charge. Theelectrical drive unit 14 meanwhile continues to draw current andtherefore discharges the capacitor 101. In so doing the voltage acrossthe capacitor 101 decays slightly and consequently the voltage at thepoint between the capacitor 101 and switch 102 rises slightly above 0V.The reservoir capacitor 107 ensures that the positive supply voltageremains constant. The differential amplifier 103 measures the voltageacross the switch 102 and the resulting measurement is converted into adigital signal by the analogue-to-digital converter 104 and passed tothe control unit 16. The small temporary voltage drop across theelectrical drive unit 14 does not have any noticeable effect on thepower transfer to the secondary devices 30.

Whilst the switch 102 is open, two separate measurements are taken attime t₁, and t₂; giving measurements V₁ and V₂ respectively, shown inFIG. 5. There is a delay t₁ after the switch is opened to allowtransient effects to stabilise. The power, P is then given by

$P = {{{CV}\frac{\mathbb{d}V}{\mathbb{d}t}} = {{{C\left( {V^{+} - \frac{\left( {V_{1} + V_{2}} \right)}{2}} \right)}\frac{\left( {V_{2} - V_{1}} \right)}{\left( {t_{2} - t_{1}} \right)}} \approx {{CV}^{+}\frac{\left( {V_{2} - V_{1}} \right)}{\left( {t_{2} - t_{1}} \right)}}}}$where V⁺ is the supply voltage, assuming V₁, V₂<<V⁺. It is advantageousto sample the voltage level at the same point in the cycle, so that theperiodic perturbation in voltage is removed (also shown in FIG. 5). Theswitch 102 is then closed again to reconnect the power supply 105 to theelectrical drive unit 14.

Incidentally, instead of the capacitor 101 an inductor could be used asan energy storage unit. In this case, the change measured by thecircuitry during the disconnection of the power supply might be a changein current, for example measured as a voltage drop across a seriesresistor.

The primary unit 10 further comprises a calibration unit 29 in thisembodiment. The calibration unit 29 stores compensation informationabout the losses in the primary unit (e.g. electrical or magneticlosses). By design, at manufacture, and/or periodically thereafter, thelosses in the primary unit may be calibrated and stored within thecalibration unit 29. The calibration unit 29 supplies the storedinformation to the control unit 16 to enable the control unit 16 tosubtract the losses from the total measurement, thus calculating anumber for the loss due to parasitic loads alone. This calibration unit29 may vary the compensation information to cope with variable losses inthe primary unit, for example losses which vary with temperature.

The secondary device 30 comprises a secondary coil 32, a rectifier 34, asecondary control unit 36, a dummy load switch 38, a dummy load 40, aload switch 42, a storage unit 44 and an actual load 46. The dummy loadswitch 38 and the load switch 42 may each be an FET, for example. Thedummy load 40 is, for example, a resistor. The storage unit 44 is acapacitor in this embodiment but an inductor could be used instead.

The actual load 46 is external to the secondary device 30 in thisembodiment and is part of the host object. It could be a battery chargecontroller for a Lithium-ion cell.

There is also a detection unit 200 for detecting a modulation imposed onthe received AC signal. To detect a frequency modulated signal, thedetection unit 200 may be a zero crossing detector which passes a signalto the control unit every time the AC signal crosses zero volts. Thecontrol unit 36 may then comprise an internal clock and counter circuit(not shown). The clock and counter circuit may be used to measure thetime interval between successive zero crossings and hence derive thefrequency of the AC signal 106 imposed by the primary unit control unit16. Hence, the secondary unit may detect a change in frequency andrespond by altering its load conditions by adjusting switches 42 and 38.

Other forms of load detection circuit 200 may comprise a thresholddetector for digital amplitude modulation or an analogue-to-digitalconverter for multiple level amplitude modulation or a phase detectorfor phase modulation or any combination thereof.

Operation of the system will now be described.

In an “operating mode” of the system, the host object incorporating thesecondary device 30 is placed on or in proximity to the primary unit 10.Switch 102 is closed. The control unit 16 applies an AC voltage signal106 to the electrical drive unit 14. The electrical drive unit 14 takesDC power from the power supply 105 and amplifies the AC voltage signal106 and applies it to the primary coil 12.

In the operating mode, the primary coil 12 generates an electromagneticfield in the vicinity of the primary unit 10. The secondary coil 32couples with this field and an alternating current is induced in thecoil by the field. The dummy load switch 38 is open and the load switch42 is closed. The alternating current induced in the secondary coil 32is rectified by the rectifier 34 and the rectified current is suppliedvia the load switch 42 to the storage unit 44 and the actual load 46. Inthis way, power is transferred inductively from the primary unit 10 tothe secondary device 30 and from there to the load 46. The storage unit44 stores energy in the operating mode.

Whilst in the operating mode, from time to time the control unit 16 inthe primary unit 10 initiates a measurement. The measurement starts withthe primary unit 10 sending a synchronisation signal to the secondaryunit 30 by applying a momentary frequency change to the AC drive voltagesignal 106. The secondary devices 30 receive the AC voltage signal andin each receiving secondary device the detection unit 200 in conjunctionwith the control unit 36 determine when the synchronisation signal hasoccurred. In response to a synchronisation signal, the secondary unitspresent alter their load conditions for a set time period and theprimary unit 10 measures the total load (power drawn) within this timeperiod.

The secondary device 30 uses the storage unit 44 to store energy fromthe primary unit 10 during normal operation. During the measurement, theactual load 46 is disconnected by opening switch 42. The energy storedin the storage unit 44 of the secondary device gradually decays asenergy is delivered to the load. Provided that the storage unit hassufficient capacity, and is sufficiently well-charged before ameasurement commences, the storage unit can deliver continuous power tothe secondary device load throughout the measurement, so the actual load46 is not interrupted.

In this embodiment the primary unit 10 initiates a series of three powermeasurements for the purposes of determining: 1) whether the there is aparasitic metal present requiring it to enter the shutdown mode toprevent overheating and 2) if there are no devices requiring any power,such that the unit can enter the standby mode. The behaviour of theprimary unit 10 and the secondary device 30 is slightly different ineach of the three measurements of the series.

During the first measurement, the secondary control unit 36 has thedummy load switch 38 open so that the dummy load 40 is not connected tothe secondary coil 32. As a result, the first measurement is a measureof the power delivered to any parasitic loads 500 from foreign objectsin the vicinity of the primary unit and any parasitic load 501 imposedby losses in the secondary device and/or its host object and any lossesin the primary unit itself. Thus, operation during the first measurementcorresponds to the steps S1 to S3 of FIG. 2 above.

During the second measurement, the secondary control unit 36 selectivelycloses the dummy load switch 38. The secondary control unit 36 decideswhether to have the dummy load switch 38 open or closed during thesecond measurement based on the power requirement of the actual load 46.If the load 46 does not require any power at the present time, forexample because it has a rechargeable battery which is presently fullycharged, then the dummy load switch 38 is kept open during the secondmeasurement. If, on the other hand, the load 46 does require power atthe present time, then the dummy load switch 38 is closed so that thedummy load 40 is connected to the primary coil 32.

The control unit 16 produces another measure of the power load duringthe second measurement. If the second power measurement is substantiallydifferent from the first power measurement, the control unit 16 detectsthat a secondary device requiring power is present in the vicinity ofthe primary unit. Thus, operation during the second measurementcorresponds to steps S11 and S12 of FIG. 3 above.

During the third measurement, the secondary control unit 36 alwayscloses the dummy load switch 38 so that the dummy load 40 is connectedto the secondary coil 32.

Another power measure is taken by the control unit 16 in the primaryunit. In this case, the measurement is the sum of the parasitic loads500, the parasitic load 501 of the secondary device and/or host object,primary unit losses, and the dummy load 40. Based on the differencebetween the first and third power measurements, the control unitcalculates the value of the total of the dummy loads 40 in all of thesecondary devices present in the vicinity of the primary unit.

The timing of the various signals and measurements is illustrateddiagrammatically in FIG. 6 (not to scale). FIG. 6( a) represents thefrequency of the drive applied to the primary coil 12, FIG. 6( b)represents the load presented by the secondary device 30, FIG. 6( c)represents the state of the switch 102 in the primary unit 10 and FIG.6( d) represents the voltage across the switch 102.

At the start of each measurement, the primary unit 10 first momentarilychanges the frequency of the drive to the primary coil 510, 511, 512,for the first, second and third measurements respectively. Then eachsecondary device 30 isolates its actual load 513, 514, 515 and dependingon the circumstances introduces a dummy load 514, 515. Within this timeframe, the switch 102 in the primary unit opens 516, 517, 518. Withinthe window of the switch opening, the voltage across the switch 106ramps up 519, 520, 521. This voltage is sampled at several points withinthis window to measure the power. On the first measurement, there is nodummy load 513, on the second measurement each device only connects thedummy load if its actual load requires power 514, on the thirdmeasurement the dummy load is always connected 515.

The secondary device 30 knows which measurement is which by the order inwhich they occur. If there has been a long gap of say a few ms since thelast synchronisation signal then the secondary device knows that it mustbe the first measurement. The secondary device can count the number ofperiods in the received alternating current to determine this. Thesecond and third measurement synchronisation signals naturally follow inthat order within a set number of cycles. In order to get more accuratemeasurements, it is possible to average each measurement over a numberof sequences.

Each dummy load 40 in a secondary device 30 in the system of thisembodiment is set to a particular value (at manufacture or duringcalibration or testing) so that the value represents the parasitic load501 imposed by the secondary device concerned and/or by its host object.

Thus, the total dummy load for all secondary devices present, ascalculated by the control unit 16, can be used by the control unit 16 assecond compensation information to compensate for the parasitic loads501 of the secondary devices present. For example, if the control unit16 detects that a substantial parasitic load 500 is present in thevicinity of the primary unit when the measured power exceeds somethreshold, the threshold may be increased by an amount dependent on thetotal parasitic load 501 of all the secondary devices present, so thatthe detection of parasitic loads 500 from foreign objects is notinfluenced by the number of secondary devices present.

FIG. 7 shows diagrammatically the load drawn for the three measurements.The load drawn is the sum of: the losses associated with the primarycoil in the primary unit (pad) 543, the parasitic load associated withforeign metal objects 542, ‘friendly parasitics’ of metal associatedwith host object (portable device) to be powered 541 and the currentload associated with all the secondary devices 540. The firstmeasurement 530 has all of these components except the load 540. If nodevices require power then the second measurement 531 will be the sameas the first measurement 530, so the primary unit can be placed into thestandby mode (step S4 in FIG. 3). However, if at least one devicerequires power then the second measurement 531 will be greater than thefirst measurement 530, and power is required. On the third measurement,each secondary device 30 connects its dummy load. The dummy load of eachdevice 40 is made equal to that device's ‘friendly parastics’. Bysubtracting the first measurement from the third measurement, the resultis the ‘friendly parasitics’ 541. The primary unit loss 543 is known(and stored in the calibration unit 29). To produce a measure of thetotal parasitic load 542 present, the calculated ‘friendly parasitics’541 and the known primary unit loss 543 may be subtracted from the firstmeasurement 530. If this figure is above a certain threshold then theunit can be placed into the shutdown mode (step S4 in FIG. 2).

A system embodying the present invention is capable of measuring loadsimposed on the primary unit sensitively, for example to within 50 mW orso. With this degree of sensitivity, it is possible to ensure that verylittle power is coupled into parasitic loads 500 such as foreignobjects.

FIG. 8 is a diagram illustrating the different modes of operation in theFIG. 4 system and the conditions for switching between these differentmodes. The three modes of operation are an operating mode, a shutdownmode and a standby mode.

In the operating mode, the primary unit is in the normal state (drivencondition) most of the time, but periodically does a sequence of threemeasurements as described above. If the result of the measurementsequence is that no secondary device requires power, the primary unitgoes into a standby mode. If the result of the measurement sequence isthat a significant parasitic load 500 is present, the primary unit goesinto a shutdown mode.

In the standby mode, the electrical drive unit 14 is stopped for most ofthe time, thus consuming little power. Periodically the primary unitenters the normal mode, then does a series of measurements in respectiveprobing periods, to check whether it should enter either the operatingmode or the shutdown mode. Otherwise it remains in the standby mode.

The shutdown mode is functionally identical to standby mode. However,the two modes may be distinguished by some user-interface feature suchas an LED to prompt the user to remove any substantial parasitic load500.

In addition to this first embodiment of the invention, there are manyother possible embodiments and combinations of features that may beutilised to advantage.

There are other inductive power transfer systems, which rather than havea single primary coil 12 have a plurality of coils, for instance asdescribed in GB-A-2398176. In such a system there may be two sets ofcoils which are arranged orthogonally to each other. They may each bedriven with the same AC voltage signal, but driven in quadrature (i.e.separated in phase by 90°), such that the induced magnetic field rotateswith time. This allows the secondary device 30 to be placed at anyorientation and still be able to receive electrical power. The presentinvention may be used in such a configuration directly. The electricaldrive unit 14 not only supplies the AC current drive to the first coil,but also to the second coil. The transmitted synchronisation signalswill be present on both coils. Furthermore, since the currentmeasurement is performed by determining the current drawn from the powersupply, the power measurement will be the total sum of all the loaddrawn regardless of what proportion is drawn by each coil. In such a 2channel rotating system, the orientation of the secondary device 30 isarbitrary. Therefore the secondary device 30 will see a +/−180° phasedifference relative to the primary unit. Therefore the secondary devices30, must each lift their load for at least ½ cycle each side of theprimary unit's measurement period.

In addition to the three measurements described, a fourth measurementmay be made. This measure is initiated by the control unit 16 in theprimary unit 10, and results in the power measurement unit 100 taking apower measurement, but without any synchronisation signal being sent tothe primary coil 12. The secondary units 30 do not alter their loadconditions and this is therefore a measurement of the power whilst inthe operating state. This measurement can be performed at any time otherthan during a measurement sequence of the first three measurements. Thisfourth measurement is used to determine if the total load drawn isgreater than the power specification of the device and hence put theprimary unit into an ‘overload state’. The ‘overload’ state isfunctionally identical to the ‘shutdown state’, but may be distinguishedby some use-interface feature such as an LED.

Another possibility is for the electrical drive unit 14 to be adapted tomodify the magnitude of its output current into the primary coil for thepurpose of varying the field magnitude of the generated magnetic field.This would allow the field magnitude to be reduced for small loads,thereby conserving electrical power. An implementation of this featureis to use the first and second measurements in a different way, not onlyto detect if devices require power, but also to set the required fieldmagnitude. Instead of switching in the dummy load 40 during the secondmeasurement if it needs power, a secondary device could switch in itsdummy load if it is not getting sufficient power. The presence of adifference between the first and second measurements would then be takenby the primary unit as a “not enough power” signal. The primary unit 10could periodically bring the field up to maximum magnitude and thengradually reduce it until the difference between the first and secondmeasurements was greater than a certain threshold (“not enough power”signal). This way the primary unit would always be operating at thelowest possible field magnitude.

In another embodiment, the secondary devices are adapted for changingthe magnitude of their dummy loads dynamically. This could be achievedfor instance by incorporating a load which may be altered in value by acontrol means. A simple example might be a resistor ladder with an arrayof switches, which may be arranged with values in binary increments. Theload may be adapted to have a continuously variable magnitude by use ofa transistor circuit or by incorporation of some other nonlinearelement. Another way of dynamically changing the load is to modulate theswitch 40 which connects the load, such that when the power measurementis averaged over the measurement time interval, the effective load isaltered. The pulse width or duty cycle could be altered to change theeffective load magnitude.

The ability to change the dummy load dynamically is useful for deviceswhose ‘friendly parasitic’ load may change. For instance a self-chargingbattery may have a different ‘friendly parasitic’ load when chargedalone, compared to when it is charged whilst connected to a mobilephone. The control unit 36 could detect whether the phone was attachedand modify the dummy load accordingly. Alternatively, the phone couldcommunicate its ‘friendly parasitics’ to the battery. Other removableattachments which contribute additional ‘friendly parasitic’ load couldalso be detected and the dummy load modified accordingly. These includefor example, but are not limited to, removable camera attachments, casesand speakers.

In addition to giving information about the load requirements andparasitic information of the secondary device 30, this method could beused such that the primary unit 10 could deduce other information aboutthe secondary device 30. For example the primary unit 10 could receiveinformation about the serial number, model number, power requirements orother information stored in the secondary device. The load could bealtered dynamically either synchronously or asynchronously to achievethis. Amplitude modulation or pulse width modulation may be used. Anumber of ‘bits’ or ‘symbols’ may be used (where a ‘symbol’ represents aplurality of amplitude levels or pulse width durations and thereforemore than one ‘bit’).

In another embodiment, the primary unit 10 could communicate informationto the secondary device 30, other than synchronisation signals, by meansof modulating the AC voltage signal 106 applied to the electrical driveunit. This information may include but is not limited to informationabout the primary unit 10, such as cost of a charge, power capability,codes; information about the location of the primary unit, such asnearby facilities; and other information such as advertising material.The secondary device 30 could receive such information by means of thedetection element 200 and the control unit 36.

It will be apparent to those skilled in the art that it is not necessaryto implement all these features simultaneously in order to gainadvantage. For instance by only using the first and second measurements,the standby detect feature may be implemented. Similarly, by only usingthe first and third measurements, the parasitic detect feature may beimplemented. By only using the fourth measurement, the overload detectfeature may be implemented. Information about the secondary device 30may be deduced by the primary unit, without implementing other features.Similarly, information may be sent from the primary unit to thesecondary device without implementing other features. Furthermeasurements may be used to implement additional features. It should beappreciated that the labelling of each measurement is purely foridentification purposes and the measurements may be performed in anyorder.

In addition to the described method of sending a synchronisation signalbefore each measurement and identifying each measurement by the order inwhich they occur, there are other methods of identifying eachmeasurement. These include but are not limited to: sending a differentsynchronisation signal before each measurement, whereby synchronisationsignals may differ in frequency offset, amplitude or phase; or sendingonly a first synchronisation signal and deducing the timing of the othermeasurements may be means of either a counter to count cycles of thereceived signal or an internal clock within each secondary device. It iseven possible to perform the measurements back to back, with nosubstantial gap between them. Alternatively measurements are initiatedby the secondary device rather than the primary unit. The secondarydevice could initiate a ‘preamble’ dynamic load modulation, which theprimary unit would detect and then synchronise on, so that its powermeasurements coincided with the timing of the secondary device adaptingits load conditions. For primary units capable of simultaneouslysupplying power to more than one secondary device, the ‘preamble’ couldinvolve the use of some unique identifier, so that each secondary devicemay be interrogated independently. ‘Preambles’ could also be used incommunication from the primary unit to the secondary device to addresseach device independently.

As described above, the dummy load may be used to represent the‘friendly parasitic’ load of the host device. Of course, the ratiobetween the dummy load value and the friendly parasitic load to becommunicated is not limited to any particular value. For instance thedummy load could be twice or three times the ‘friendly parasitic’ valueor a noninteger multiple of the value. The primary unit can deduce thetotal ‘friendly parasitic load’ so long as it knows what the ratio is.Furthermore, if a device does not have any significant ‘friendlyparasitic’ load, it may be desirable to ‘assign’ it a particular value,so that it may be used for indicating whether the device requirescharge. It may be desirable to use more than one dummy load. A firstdummy load may be used for the second measurement and a second dummyload may be used for the third measurement. The first dummy load wouldbe used for standby detection and the second dummy load would berepresentative of the ‘friendly parasitics’. This is particularlyadvantageous if the secondary devices have widely varying parasiticloads. The first dummy load could also be used for determining the powerrequirements of the secondary devices requiring charging, rather justmaking a standby decision. The dummy load value would be adapted to berepresentative of the power requirements of that particular device. Thefirst and second dummy loads may be implemented by a single dynamicallyvariable dummy load described above, or fixed loads may be used, or acombination of the two.

In addition to the power measurement method and apparatus described, itwill be appreciated that there are many methods which can be used todetect the load on the primary coil or coils. The simplest powermeasurement may comprise inserting a series resistor on one supply rail.The voltage could be measured across that resistor and the power deducedfrom the observed voltage and the known resistor value. With such amethod it may be desirable to incorporate a switch across the resistor,so that during periods outside of the measurement time, the resistor maybe short circuited, such that there is no unnecessary power dissipationin the resistor.

Another method of power measurement is to measure the power within theelectrical drive unit. For instance, it is desirable for the electricaldrive to the coil or coils to be regulated by means of a feedbackcircuit. The feedback signal may be used to derive a power measurement.

It is also possible to combine the functions of sending asynchronisation signal and power measurement within a single element asdescribed for instance, in the applicant's copending application GB0410503.7 filed on 11 May 2004, from which the present applicationclaims priority. In that system, the power measurement involvesdisconnecting the power to the primary coils and detecting the decay inthe undriven resonant circuit. The act of disconnecting the power to theprimary coil 12 also has the effect of modulating the signal in theprimary coil and as a result the signal received in the secondary device30.

FIG. 9 shows a second embodiment of a power transfer system according tothe present invention. This embodiment differs from the first embodimentof FIG. 4 mainly in the way in which the power measurements are carriedout. The primary unit 110 comprises a primary coil 112, an electricaldrive unit 114, a control unit 116 and a decay measurement unit 118. Theelectrical drive unit 114 in this embodiment has a conventional halfbridge configuration in which a first switch 120 is connected between afirst power supply line of the primary unit and an output node of theelectrical drive unit, and a second switch 121 is connected between theoutput node and a second power supply line of the primary unit. Thefirst and second switches 120 and 121 may, for example, be field-effecttransistors (FETs).

The electrical drive unit 114 also comprises a drive controller 119which applies control signals to the switches 121 and 122 to turn themon and off. The drive controller 119 has a control input connected to anoutput of the control unit 116. The output node of the electrical driveunit 114 is connected via a capacitor 117 to one side of the primarycoil 112.

The control unit 116 is a microprocessor in this embodiment.Alternatively, an ASIC could be used to implement the control unit 116,as well as some or all of the other circuit elements of the primaryunit.

The decay measurement unit 118 comprises a resistor 125 which has afirst node connected to one side of a switch 128 and a second nodeconnected to the second power supply line. The resistor 125 is alow-value resistor. The decay measurement unit 118 further comprises anoperational amplifier 126 having an input connected to the first node ofthe resistor 125. The decay measurement unit 118 also comprises ananalog-to-digital converter (ADC) 127 connected to an output of theoperational amplifier 126. An output of the ADC 127 is connected to ameasurement input of the control unit 116.

The other side of the switch 128 is connected to the other side of theprimary coil 112. A snubber unit 122 is connected in parallel with theswitch 128. The snubber unit 122 comprises a capacitor 123 and aresistor 124 connected in series with one another. The calibration unit129 is the same as the calibration unit 29 in FIG. 4.

Each secondary device in this embodiment can be substantially the sameas the secondary device 30 in FIG. 4, and accordingly a descriptionthereof is omitted here and no secondary device is illustrated in FIG.9.

Operation of the FIG. 9 system will now be described with reference toFIG. 10.

Initially, the system has a normal state in which the control unit 116causes the electrical drive unit 114 to apply drive signals to theprimary coil 112 to cause it to oscillate. It will be appreciated thatin the operating mode, the system is in this state for almost all thetime. The switch 128 is closed, and the circuit including the capacitor117 and primary coil 112 forms a resonant tank.

The next state is a “snub” state. The application of drive signals tothe primary coil 112 by the electrical drive unit 114 is suspended underthe control of the control unit 116. The drive controller 119 closes theswitch 121. The control unit 116 also opens the switch 128 at a timewhen most of the energy in the resonant tank resides in the capacitor117. The opening of the switch 128 brings the snubber unit 122 in serieswith the resonant tank. The snubber unit 122 quickly dissipates anyenergy which remains in the primary coil 112, stopping it fromresonating within one cycle or so. Most of the energy stored in theresonant tank is left in the capacitor 117. The sudden cessation ofcycles is detected by the detection unit 200 and the secondary controlunit 36 in the secondary device 30. The secondary control unit 36 opensthe load switch 42. Incidentally, it will be appreciated that hedetection unit 200 in FIG. 4 needs to be modified to detect the suddencessation of cycles in the snub state in the present embodiment. Athreshold detector (as mentioned above) may be used as the detectionunit in the present embodiment.

In this embodiment, therefore, the snub state is used as thesynchronisation signal to the secondary device, though the other forms(e.g. frequency or phase modulation) could also be used. It is notalways necessary to have a synchronisation signal before everymeasurement as described above.

The system then enters the decay state from the snub state. The controlunit 116 closes the switch 128, removing the snubber unit 122 from theresonant tank, and thus allowing the energy in capacitor 117 to flowagain within the resonant tank. In the decay state, the resonant tankoperates in an undriven resonating condition. Energy stored in theresonant tank decays over the course of time in the decay state. In thisembodiment, the decay measurement unit 118 measures the energy decay inthe resonant tank by measuring the current flowing through the primarycoil 112. The same current flows through the resistor 125 and generatesa voltage at the first node of that resistor. This voltage is bufferedby the operational amplifier 126 and converted into a digital signal bythe ADC 127. The resulting digital signal is applied to the measurementinput of the control unit 116.

FIG. 10 shows how the current flowing through the primary coil 12 variesin the normal, snub and decay states which occur during a powermeasurement. In this embodiment, the digital signals representing thecurrent flowing in the primary coil within a measurement period arereceived and processed within the control unit 16 to calculate a measureof the rate of energy decay in the resonant tank.

An equation describing the energy stored in the resonant tank, atresonance, is:

$E = {{\frac{1}{2}L\;{\hat{I}}^{2}} = {\frac{1}{2}C\;{\hat{V}}^{2}}}$where E is the energy, L the inductance, Î is the peak current, C is thecapacitance and {circumflex over (V)} is the peak voltage.

Therefore the energy stored in the resonant tank of the primary unit atany given moment can be calculated if the inductance and peak currentare known, or if the capacitance and peak voltage are known, orcombinations thereof. Typically the capacitance is known by design, thepeak current and voltage can be measured by suitable circuitry, and theinductance can be deduced by observing the natural resonant frequencyduring the measurement and applying the formula:

$L = \frac{1}{4\pi^{2}f^{2}C}$

The power measure, P, is given by the rate of decay of energy (and thusthe loss) from the resonant tank and can be calculated by measuring E₁at time T₁ and E₂ at another time T₂.

$P = \frac{E_{2} - E_{1}}{T_{2} - T_{1}}$

Since at resonance the voltage and current in the resonant tank will be90 degrees out of phase with one another, a convenient method of readingthe peak voltage of one is to trigger the measurement on thezero-crossing of the other.

A second method for detecting a shutdown condition in accordance withthe present invention will be described with reference to FIG. 11. Thismethod can be used in the FIG. 1 system.

When the FIG. 1 system is in use, from time to time each secondarydevice which is in the power-requiring state supplies information to theprimary unit relating to its own power requirement. The powerrequirement information may take many different forms. For example, theinformation may comprise a binary part used to indicate either “no powerrequired” or “power required”. In this case, in the event that a binarypart is “power required”, supplemental information may be provided bythe secondary device to indicate the amount of power required.Alternatively the power requirement information may simply berepresentative of the amount of power required, and “0” may betransmitted if the device requires no power at all. It is also possiblethat the power requirement of the secondary device may already be knownto the primary unit. For example, it may be known that all secondarydevices of a certain type will have a particular power requirement. Inthis case, the power requirement information may simply be a code (orsome other identifying information) indicating the type of the secondarydevice.

All of the secondary devices may supply the power requirementinformation simultaneously to the primary unit. Alternatively, eachsecondary device supplies its power requirement information individuallyin turn to the primary unit.

The power requirement information supplied by each secondary device thathas the power-requiring state is received by the primary unit.

In step S22, the control unit 16 in the primary unit causes the powermeasurement unit 100 to measure the power drawn by the secondary devicesfrom the primary unit as described previously. In practice, the measuredpower will also reflect any losses in the system.

In step S23 the control unit 16 determines whether inductive powersupply from the primary unit should be restricted or stopped independence upon the measured power in step S22 and the power requirementinformation received in step S21. For example, the control unit 16calculates a sum of the respective power requirements of all of thesecondary devices that are in the power-requiring state. This sum iscompared with the measured power found in step S22. If the measuredpower exceeds the sum of the power requirements by more than a thresholdvalue, then the control unit determines that a substantial parasiticload must be present in the vicinity of the primary unit. In that case,processing proceeds to step S24 in which the primary unit enters theshutdown mode and inductive power supply from the primary unit isrestricted or stopped. As described previously in relation to the methodof FIG. 2, the system may be reset manually or automatically in stepS25.

If in step S23 the control unit 16 determines that the power supply neednot be restricted or stopped, then processing returns to step S21, forexample after a predetermined time interval.

To compensate for the losses in the primary unit and/or secondarydevices, the shutdown threshold employed in step S23 may be adjusted.One way in which this can be done is for each secondary device (whetheror not in the power-requiring state) to also supply to the primary unitinformation relating to its “friendly parasitic” load. Similarly, lossesas in the primary unit may be accounted for using a calibration unit asdescribed with reference to FIG. 4.

FIG. 12 shows parts of a power transfer system according to a thirdembodiment of the present invention. This system implements the shutdowndetection method of FIG. 11 using an RFID communication method.

The FIG. 12 system comprises a plurality of secondary devices 600 ₁, 600₂, . . . , 600 _(n). The system of FIG. 12 also comprises a primary unit700. The primary unit 700 comprises an RFID unit 710, a control unit 720and a power measurement unit 730. The control unit 720 correspondsgenerally to the control unit 16 described previously with reference toFIG. 1, and the power measurement unit 730 corresponds generally to thepower measurement unit 100 described in reference to FIG. 1.

The features of the secondary device 600 are generally the same as thoseof the secondary device 30 in FIG. 4 except that the elements 38, 40,42, 44 and 200 may be omitted. Instead of these elements, each secondarydevice 600 comprises its own load measuring unit 610 and an RFID unit620. The load measuring unit 610 measures the power being supplied tothe actual load (46 in FIG. 4) of that secondary device. For example,the load measuring unit 610 may measure the current and/or voltage beingsupplied to the actual load 46 and may possibly integrate these measuresover time for averaging purposes. For example, the averaging period maybe ten seconds.

The RFID unit 620 in each secondary device is capable of communicatingwith the RFID unit 710 in the primary unit 700 using an RFID link 630.The load measure produced by the load measuring unit 610 in eachsecondary device is supplied to the RFID unit 620 in the device and thentransmitted via the relevant RFID link 630 to the RFID unit 710 in theprimary unit. For example, the RFID unit 710 may poll the RFID unit 610in each of the secondary devices from time to time. In response, theRFID unit 620 that has been polled transmits its load measure. This loadmeasure corresponds to the power requirement information of step S21 inFIG. 11.

The power measurement unit 730 in the primary unit also measures thepower being drawn by the secondary devices from the primary unit, as instep S22 in FIG. 11. Then, the control unit 720 determines, independence upon the measured power and the sum of the received loadmeasures from the secondary devices, whether or not the power supply tothe secondary devices should be restricted or stopped. In particular, ifthe measured power from the power measurement unit 730 exceeds the sumof the load measures from the secondary devices by more than a shutdownthreshold, then the control unit 720 concludes that a substantialparasitic load must be present and places the primary unit in theshutdown mode, as in step S24 in FIG. 11.

It is also possible for the load measure produced by each secondarydevice to represent a total load from the secondary device including theamount of power required by the actual load and any friendly parasiticload of the secondary device/host object. If the actual load requires nopower the load measure may change to represent only the friendlyparasitic load.

Some anti-collision or collision avoidance technique is necessary in theFIG. 12 embodiment. In one known collision avoidance technique, everyRFID unit 620 has an unique code (or one which will be practicallyunique based on statistics). The RFID unit 710 in the primary unit wouldsend out a signal requesting that all RFID units 620 within a certainrange reply. The RFID units 620 send out their reply in code (e.g.Manchester code), so that the RFID unit 710 can tell if more than onedevice has replied. The primary unit gradually narrows down the rangeuntil it can uniquely identify the code of each device present.Typically, it may halve the range of codes in each iteration to home inquickly.

It will be appreciated that instead of RFID, any suitable communicationlink can be used to permit each secondary device to communicate itspower requirement information to the primary unit. For example, infraredor ultrasound communication could be used. Alternatively, each secondarydevice could vary the load it imposes on the primary unit to communicatethe power requirement information. For example, each secondary devicecould impose a dummy load representing the amount of power required byits actual load. In this technique, all the secondary devices in thepower-requiring state can impose their respective dummy loadssimultaneously, so that the primary unit can directly derive the sum ofthe power requirements of all the secondary devices in one measurement.Alternatively, the dummy load may represent a total load from thesecondary device including the amount of power required by the actualload and any friendly parasitic load of the secondary device/hostobject.

1. A method of controlling inductive power transfer in an inductivepower transfer system comprising a primary unit operable to generate anelectromagnetic field and at least one secondary device, separable fromthe primary unit, and adapted to couple with said field when thesecondary device is in proximity to the primary unit so that power canbe received inductively by the secondary device from the primary unitwithout direct electrical conductive contacts therebetween, which methodcomprises: receiving, in the primary unit, information from thesecondary device relating to a power required by the secondary deviceor, if there is more than one secondary device, a combined powerrequired by the secondary devices; determining in the primary unit, independence upon the received information, whether there is parasiticmetal in the electromagnetic field by determining if there is asubstantial difference between a power drawn from the primary unit and apower required by the secondary device or, if there is more than onesecondary device, a combined power required by the secondary devices;following such determination of parasitic metal in the electromagneticfield, restricting or stopping the inductive power supply from theprimary unit; employing, when carrying out said determination, firstcompensation information relating to losses in the primary unit itselfso as to compensate for said losses; employing, when carrying out saiddetermination, second compensation information relating to a parasiticload imposed on the primary unit by the secondary device so as tocompensate for said parasitic load of the secondary device; wherein thesecondary device communicates its said second compensation informationdirectly to said primary unit or communicates to the primary unit otherinformation from which the primary unit derives said second compensationinformation; wherein the secondary device communicates its said secondcompensation information or its said other information to the primaryunit by varying a load imposed by it on the primary unit; and whereinthe secondary device has a dummy load, representative of its saidparasitic load, which it imposes on said primary unit to vary the loadimposed by it on the primary unit.
 2. A method of controlling inductivepower transfer in an inductive power transfer system comprising aprimary unit operable to generate an electromagnetic field and at leastone secondary device, separable from the primary unit, and adapted tocouple with said field when the secondary device is in proximity to theprimary unit so that power can be received inductively by the secondarydevice from the primary unit without direct electrical conductivecontacts therebetween, which method comprises: receiving, in the primaryunit, information from the secondary device relating to a power requiredby the secondary device or, if there is more than one secondary device,a combined power required by the secondary devices; determining in theprimary unit, in dependence upon the received information, whether thereis parasitic metal in the electromagnetic field by determining if thereis a substantial difference between a power drawn from the primary unitand a power required by the secondary device or, if there is more thanone secondary device, a combined power required by the secondarydevices; following such determination of parasitic metal in theelectromagnetic field, restricting or stopping the inductive powersupply from the primary unit; and wherein the electromagnetic field isgenerated by a primary coil driven by an electrical drive unit,electrical power for the drive unit is supplied from a power supply ofthe primary unit to a power input of the drive unit, and the power drawnfrom the primary unit is measured by temporarily disconnecting the powersupply and detecting a change at said power input during thedisconnection.
 3. A method as claimed in claim 2, further comprisingstoring energy in an energy storage unit connected to said power inputso that power can continue to be supplied to said power input whilstsaid power supply is disconnected.
 4. A method of controlling inductivepower transfer in an inductive power transfer system comprising aprimary unit operable to generate an electromagnetic field and at leastone secondary device, separable from the primary unit, and adapted tocouple with said field when the secondary device is in proximity to theprimary unit so that power can be received inductively by the secondarydevice from the primary unit without direct electrical conductivecontacts therebetween, which method comprises: receiving, in the primaryunit, information from the secondary device relating to a power requiredby the secondary device or, if there is more than one secondary device,a combined power required by the secondary devices; determining in theprimary unit, in dependence upon the received information, whether thereis parasitic metal in the electromagnetic field by determining if thereis a substantial difference between a power drawn from the primary unitand a power required by the secondary device or, if there is more thanone secondary device, a combined power required by the secondarydevices; and following such determination of parasitic metal in theelectromagnetic field, restricting or stopping the inductive powersupply from the primary unit, wherein the electromagnetic field isgenerated by a primary coil, and the method further comprises: causing acircuit including said primary coil to operate, during a measurementperiod, in an undriven resonating condition in which the application ofdrive signals to the primary coil is suspended so that energy stored inthe circuit decays over the course of said period; taking one or moremeasures of such energy decay during said period and employing said oneor more measures to measure said power drawn from the primary unit.